1. Field of the Invention
The present invention relates to a low-pass filter composed of an LC filter.
2. Description of the Related Art
Low-pass filters are one of electronic components used in communication apparatuses. Among known low-pass filters is one that is composed of an LC filter formed using an inductor and a capacitor.
For example, the following three performance characteristics are demanded of low-pass filters: first, sufficiently small insertion loss and sufficiently high return loss in the pass band; second, sufficiently high insertion loss in the stop band; and third, an abrupt increase in insertion loss with an increase in frequency in a frequency region within the stop band near the cut-off frequency.
A low-pass filter having such a characteristic that the frequency characteristic of the insertion loss forms one or more attenuation poles in the stop band is known to be capable of achieving the aforementioned first to third performance characteristics. Such a low-pass filter is disclosed in, for example, US 2017/0170798 A1.
The low-pass filter disclosed in US 2017/0170798 A1 includes first and second input/output terminals, first and second LC parallel resonators, first to third capacitors, and an inductor. The first and second LC parallel resonators are connected in series and provided between the first and second input/output terminals. One end of the first capacitor is connected to the first input/output terminal. One end of the second capacitor is connected to the second input/output terminal. One end of the third capacitor is connected to the connection point between the first and second LC parallel resonators. One end of the inductor is connected to the other end of each of the first to third capacitors. The other end of the inductor is connected to the ground.
An example of specific performance characteristics demanded of low-pass filters is that the return loss in a first frequency band within the pass band be not less than a first value while the insertion loss in a second frequency band within the stop band be not less than a second value.
In general, when adjusting the characteristics of a low-pass filter composed of an LC filter to satisfy requirements for certain performance characteristics, the inductance(s) of one or more inductors and/or the capacitance(s) of one or more capacitors may be adjusted.
To adjust the characteristics of the low-pass filter disclosed in US 2017/0170798 A1 to satisfy requirements for certain performance characteristics, the capacitance(s) of at least one of the first to third capacitors may be adjusted, for example.
In the low-pass filter disclosed in US 2017/0170798 A1, changing the capacitance(s) of at least one of the first to third capacitors would cause the frequency characteristic of the insertion loss in the stop band (hereinafter referred to as attenuation characteristic) and the frequency characteristic of the return loss in the pass band (hereinafter referred to as reflection characteristic) to change simultaneously. For this low-pass filter, it is thus difficult to adjust the characteristics to satisfy requirements for certain performance characteristics.